The invention concerns a fermentation device for biological degradation of a substrate containing organic material and for recovering biogas generated in the degradation process, comprising an elongate closed container with an inlet opening for the substrate at one end and an outlet opening for the treated substrate at the opposite end of the container as well as at least one removal opening for biogas, wherein the container has several reaction cells that contain, respectively, several individually driven mixing units for circulating the substrate, with mixing impellers moved on a circular path and with mixing unit shafts that are positioned transversely to the longitudinal extension of the container, as well as a method for operating the fermentation device.
When fermenting a substrate containing organic material, for example, biowaste, residues of food production, or renewable raw materials, a differentiation is made between the so-called wet fermentation and the so-called dry fermentation. While in wet fermentation a flowable suspension of the substrate is produced that is treated in a biogas reactor, for example, a loop reactor with internal loop flow, the substrate in dry fermentation is introduced, for example, at one end into a closed horizontal container, is circulated by means of a mixing device with generation of biogas, and the treated substrate is removed at the other end of the container.
The methods of dry fermentation are used alternatively to those of wet fermentation when the substrate to be treated anaerobically has a very high solids contents and therefore the required dilution to the operating range of the wet method with a solids concentration of less than approximately 15% is uneconomical or when the frequently required cost-intensive removal of unwanted material in wet methods is not required for other process-technological reasons. Dry fermentation is carried out preferably for solids contents of equal to or less than 35% at the fermenter inlet and can be performed in upright or horizontal reactors.
Advantages of dry fermentation when used in waste and residual waste treatment are the comparatively minimal pretreatment and conditioning expenditures for the received feedstock before fermentation. After removal of coarse unwanted material and subsequent commination of the batch for fermentation to 20 to 60 mm, generally there is no need for further removal of unwanted material, for example, wood, plastics, glass as well as sand and stones.
This minimal technological expenditure for initial treatment is counterbalanced by possible problems in the fermenter itself. As a result of the biochemical conversion of organic compounds in the fermentation medium across the fermenter length, the solids concentrations are reduced between the inlet opening and the removal opening from approximately 35% dry material to less than 20% dry material and the viscosity is reduced from 30,000 to 60,000 mPas down to less than 5,000 mPas. While in the solids-rich feedstock the afore mentioned unwanted materials capable of sedimentation and floating have been distributed substantially stationarily, they are now mobilized by the changing material properties of the fermentation medium in the fermenter which tends to result in the formation of sinking and floating layers. The mixing devices that are usually provided in the fermenter for improving the material transfer and degassing of the medium further enhance this fractioned separation.
Therefore, especially in horizontal dry fermenters there are also special devices provided for the transport and removal of sediments in addition to the mixing devices.
For example, in EP 0 617 120 B1 special scrapers are disclosed that transport the sediments at the bottom of the fermenter by means of movements transmitted by push rods to the end of the reactor in the direction of the thinning medium.
In this known construction, it is thus necessary to provide separate devices for mixing and for removal of unwanted materials; this increases the technological expenditure and the operational expenditure. In particular, servicing and elimination of malfunctions cause significant problems because they may require removal of the fermenter from the site.